Temperature sensor based capture detection for a pacer

ABSTRACT

A pacemaker system which includes a fast responding temperature sensor located near the distal end of a transvenous pacing lead generates a cycle-by-cycle variation of temperature indicative of the mechanical pumping action of the heart. A timer is used to define a detection window after the generation of a pacing pulse. The occurrence of a detected depolarization within the detection window indicates that the pacing pulses capture the heart.

BACKGROUND OF THE INVENTION

This invention relates to the implantable pacemakers and moreparticularly to a system for detecting the evoked response of thecardiac tissue shortly after the application of an electricalstimulation pulse.

DESCRIPTION OF THE PRIOR ART

The cardiovascular system provides oxygenated blood to variousstructures of the body. The body's demand for oxygenated blood isreflected by the rate at which the sinus node of the heart beats. Theelectrical signal generated by the sinus node causes the atria or upperchambers of the heart to contract, forcing blood into the lower chambersor ventricles of the heart. After a brief delay, the lower chambers ofthe heart contract forcing the blood throughout the body. Thecontraction of the ventricles proceeds in an organized fashion which isreflected by the passage of a depolarization wave front through theheart muscle.

Various disease mechanisms cause conduction disturbances which interferewith the natural conduction system of the heart. A variety ofimplantable medical devices have been developed to treat theseabnormalities. The bradycardia pacemaker is an example of one suchimplantable medical device which supplies therapeutic stimulation to theheart to compensate for these conduction defects.

The modern pacer system comprises a catheter or lead system, and a pulsegenerator or pacer. The lead system is passed through a vein into theright ventricle of the heart. There are two forms of lead systems incommon use. The first form is a unipolar lead which has a tip electrodelocated proximate the distal end of the lead. The pacemaker housing orcan forms a reference electrode in this configuration. The second formof lead system is the bipolar lead which includes a tip electrode usedin conjunction with a ring electrode located near the tip electrode. Ineither case, the distal end of the lead carries a tip electrode whichcontacts the myocardium. The proximal end of the lead is connected tothe pulse generator. The pulse generator is usually implantedsubcutaneously outside the rib cage.

The first pacemakers paced the heart at a metronomic rate independent ofthe heart's underlying rhythm. Such pacemakers are typified by U.S. Pat.No. 3,057,356 to Greatbatch. One problem with such pacemakers is thatthey may compete with the heart's underlying rhythm and provoke lethalarrhythmias.

The demand pacer was introduced to overcome this defect. This form ofpacer contains circuitry to detect a depolarization of the cardiactissue. The circuitry for performing this function is referred to as asense amplifier in the art. The function of the sense amplifier is togenerate a sense event signal which is used by the escape interval timerof the pacer to synchronize the pacer to the heart's rhythm. Inoperation the pacer escape interval timer is set to a nominalstimulation rate (standby rate) which reflects the lowest permissibleheart rate. If the underlying heart rate is above the standby rate, thepacer detects the cardiac depolarization and prevents the delivery ofpacing stimuli. This form of pacer is now classified as a VVI mode pacerand is taught to the art by U.S. Pat. No. 3,345,990 to Berkovitz. Theefficacy and safety of this pacing modality requires reliable sensing ofheart activity.

A DDD mode pacemaker senses electrical signals in both the atrium andventricle of the patient's heart, and delivers an atrial pacing stimulusin the absence of signals indicative of natural atrial contractions, andventricular pacing stimuli in the absence of signals indicative ofnatural ventricular contractions. The delivery of each pacing stimulusby a DDD pacemaker is synchronized with prior sensed or paced events.

Pacemakers are also known which respond to other types ofphysiologic-based signals, such as signals from sensors for measuringtemperature or oxygen inside the patient's heart or measuring the levelof a patient's activity. These rate-responsive pacemakers are labeledVVIR for a single chamber version or DDDR for a dual chamber version.Examples of these rate-responsive pacemakers are as described by thefollowing U.S. Pat. Nos.:

4,428,378 to Anderson et al (activity),

4,543,954 to Cook et al (temperature), and

4,750,495 to Moore et al (oxygen).

The temperature and oxygen sensor signals typically used for arate-responsive function utilize a low pass filtered long-term averagedsignal. High-frequency components are treated as noise, see for example,"Continuous Thermal Measurement of Cardiac Output," Phillip et al., IEEETransaction on Biomedical Engineering, Volume BME31, No. 5, May 1984.This reference demonstrates a cardiac response with a fast-actingtemperature sensor; filtering out and using the low-frequency component(under 0.04 Hz) for improved thermal dilution techniques for continuouscardiac output measurement.

In an effort to extend the useful operating life of pacemakers and toallow extraction of useful diagnostic information, it has been common inrecent years to provide a programmable output stimulation pulse whichpermits the physician to select an output pulse energy which is known tobe sufficient to capture the heart but which is below the maximumobtainable output energy. In operation the physician can conservebattery power and thus extend the useful life of the pacer by selectingan output pulse energy just above the stimulation threshold of thepatient's heart.

It has also been proposed to automatically adjust the output energylevel. U.S. Pat. No. 4,305,396 issued to Whitkampf, et al. teaches apacer in which the pacemaker has its output energy automaticallycontrolled in response to the detection of driven R-waves and its pacingrate varied as a function of the energy required to capture the heart.Practical realization of such systems has not occurred, because thepacer output stimulus which is delivered to the lead system is manyorders of magnitude larger than the electrical signal generated by theheart and can mask detection of the evoked response or stimulatedR-wave. However, this reference illustrates a longstanding desire for apractical detector system capable of reliably sensing an evokedresponse.

SUMMARY OF THE INVENTION

In contrast to the approach taken by the prior art, the presentinvention utilizes a fast responsive temperature sensor located proximalto the distal end of a transvenous pacing lead to monitor the intrabeatvariation of venous blood temperature due to the pumping action of theheart. A signal indicative of the mechanical pumping action of the heartis detected and evaluated for sufficient magnitude to indicate acontraction subsequent to the occurrence of an output pacing stimulus.

Timer circuits coupled to the stimulating and detection circuitscooperate to define a capture detection window, initiated shortly afterthe delivery of a pacing pulse. The cardiac signal indicative of thecardiac pumping action occurring within this time window is defined asan evoked response. The occurrence or non-occurrence of an evokedresponse following a pacing pulse may be used to provoke a statetransition in the pacer to alter its operation. For example, theamplitude or pulse width of the pacing pulse may be adjusted to providereliable pacing at the minimum appropriate pulse energy level.Alternatively, the occurrence of the evoked response may be used fordiagnostic purposes.

In a preferred embodiment of the pacer described herein, the detectionof an evoked response is used to control the stimulation energydelivered by the pacer output stage. In general, the auto thresholdpacer disclosed will minimize its output energy to maximize pulsegenerator longevity.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will be bestappreciated with reference to the detailed description of a specificembodiment of the invention, which follows, read in conjunction withaccompanying drawings wherein:

FIG. 1 is a diagram showing the placement in a patient of a pacemaker inaccordance with the present invention;

FIG. 2 is a block diagram of the circuitry of a pacemaker in accordancewith an embodiment of the present invention;

FIG. 3 is a timing diagram which reflects function of the invention; and

FIG. 4 is a machine description of the procedure for detecting theevoked response.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, reference is made to an illustrativeembodiment for carrying out the invention. It is understood that otherembodiments may be utilized without departing from the scope of theinvention. For example, the invention is disclosed in the context of aVVI modality pacer for treating bradycardia. It should be appreciatedthat the technique for evoked response detection could also be appliedto a dual chamber device where capture detection is used to control theenergy of the pacing stimuli delivered to the atria. In a similarfashion the ability to detect the evoked response will find utility intachyarrhythmia pacers where direct evidence that capture has occurredcan be used as feedback to control the delivery of tachyarrhythmiatherapies.

The present invention will now be more fully described with reference tovarious figures of the drawings, FIG. 1 showing generally how apacemaker 10 in accordance with the present invention may be implantedin patient 12. A pacemaker lead 14 is electrically coupled to pacemaker10 and extends into the patient's heart 16 via a vein 18. The distal endof the lead 14 includes one or more exposed conductive electrodes forreceiving electrical cardiac signals and for delivering electricalpacing stimuli to the patient's heart 16. In accordance with theinvention to be hereinafter described, the distal end of the pacing lead14 also incorporates a temperature transducer (not shown in FIG. 1 dueto the small scale of that figure) for producing electrical signalsrepresentative of the temperature of the blood contained within theright ventricle of the heart 16.

Turning to FIG. 2, a block diagram of pacemaker 10 from FIG. 1 is shown.Although the present invention is described in conjunction with thepacemaker 10 having a microprocessor-based architecture, it will beunderstood that it could be implemented in any logic-based, customintegrated circuit architecture, if desired. It will also be understoodthat the present invention may be utilized in conjunction with otherimplantable medical devices, such as cardioverters, defibrillators,diagnostic monitoring devices, cardiac assist systems, and the like.

In the embodiment shown in FIG. 1, pacemaker 10 includes an activitysensor 20, which may be, for example, a piezoelectric element bonded tothe inside of the pacemaker's housing. Sensor 20 provides a sensoroutput which varies as a function of a measured parameter that relatesto the metabolic requirements of patient 12. In addition, pacemaker 10includes a temperature sensor 22 disposed at the distal end of lead 14,as previously noted, which may be similarly used to ascertain themetabolic requirements and/or cardiac output of patient 12. Theconstruction of the lead is not critical to the invention, varioussuitable lead arrangements being known to those skilled in the art. Seefor example, U.S. Pat. No. 4,543,954 to Cook, et al. and incorporatedherein by reference.

Pacemaker 10 is schematically shown in FIG. 2 to be electrically coupledby a pacing lead 14 to a patient's heart 16. Lead 14 includes anintracardiac electrode 24 and temperature sensor 22 located near itsdistal end and positioned within the right ventricular chamber heart 16.Lead 14 can carry either unipolar or bipolar electrodes as is well knownin the art. In the presently disclosed embodiment, lead 14 which couplespacemaker 10 to the ventricular endocardium can comprise asteroid-tipped, unipolar lead with an integral temperature transducer ofthe type describe in the aforementioned reference. Electrode 24 iscoupled by a suitable lead conductor 14a through input capacitor 26 tonode 28 and to input/output terminals of an input/output circuit 30.Output from the first sensor 20 is coupled to input/output circuit 30.Output from temperature sensor 22 is also coupled to input/outputcircuit 30 by a suitable lead conductor 14b.

Input/output circuit 30 contains the analog circuits for interface tothe heart 16, first sensor 20, temperature sensor 22, and antenna 52, aswell as for the application of stimulating pulses to heart 16 to controlits rate as a function thereof under control of the software-implementedalgorithms in a microcomputer circuit 32.

Microcomputer circuit 32 comprises an on-board circuit 34 and anoff-board circuit 36. On-board circuit 34 includes a microprocessor 38,a system clock circuit 40, and on-board RAM 42 and ROM 44. Off-boardcircuit 36 includes an off-board RAM/ROM unit 46. Microcomputer circuit32 is coupled by data communication bus 48 to a digital controller/timercircuit 50. Microcomputer circuit 32 may be fabricated of customintegrated circuit devices augmented by standard RAM/ROM components.

It will be understood that the electrical components represented in FIG.2 are powered by an appropriate implantable battery power source, notshown, in accordance with common practice in the art.

An antenna 52 is connected to input/output circuit 30 for purposes ofuplink/downlink telemetry through RF transmitter/receiver (RF TX/RX)unit 54. Telemetering both analog and digital data between antenna 52and an external device, such as an external programmer (not shown), isaccomplished in the presently disclosed embodiment by means of datafirst being digitally encoded and then pulse position modulate on adamped RF carrier, as substantially described in co-pending U.S. patentapplication Ser. No. 07/468,407, filed on Jan. 22, 1990, entitled"Improved Telemetry Format," which is assigned to the assignee of thepresent invention and which is incorporated herein by reference.

A crystal oscillator circuit 56, typically a 32,768 Hzcrystal-controlled oscillator, provides main timing clock signals todigital controller/timer circuit 50. A Vref/Bias circuit 58 generates astable voltage reference and bias currents for the analog circuits ofinput/output circuit 30. An analog to digital converter/multiplexor(ADC/MUX) unit 60 digitizes analog signals and voltages to provide "realtime" telemetry of temperature and intracardiac signals and batteryend-of-life (EOL) replacement function. A power-on reset (POR) circuit62 functions as a means to reset circuitry and related functions to adefault condition upon detection of a low battery condition, which willoccur upon initial device power-up or will transiently occur in thepresence of electromagnetic interference, for example.

The operating commands for controlling the timing of pacemaker 10 arecoupled by bus 48 to digital controller/timer circuit 50 wherein digitaltimers and counters are employed to establish the overall escapeinterval of the pacemaker, as well as various refractory, blanking, andother timing windows for controlling the operation of the peripheralcomponents within input/output circuit 30.

Digital controller/timer circuit 50 is coupled to a sense amplifier 64and an electrogram amplifier 66 for receiving amplified and processedsignals picked up from electrode 24 through lead conductor 14a andcapacitor 26 representative of the electrical activity of the patient'sheart 16. Sense amplifier 64 amplifies sensed electrical cardiac signalsand provides this amplified signal to peak sense and thresholdmeasurement circuitry 65, which provides an indication of peak sensevoltages and the measured sense amplifier threshold voltage on multipleconductor signal path 67 to digital controller/timer circuit 50. Theamplified sense amplifier signal is also provided to a comparator 69.The electrogram signal developed by EGM amplifier 66 is used on thoseoccasions when the implanted device is being interrogated by an externalprogrammer (not shown) in order to transmit by uplink telemetry arepresentation of the analog electrogram of the patient's electricalheart activity as described in U.S. Pat. No. 4,556,063, issued toThompson et al., assigned to the assignee of the present invention andincorporated herein by reference. Input/output circuit 30 furtherincludes sensitivity control circuitry 75 coupled between digitalcontroller/timer circuit 50 and sense amplifier circuit 64. Sensitivitycontrol circuit 75 controls the sense amplifier gain and thus thesensing threshold of sense amplifier 64 as instructed by digitalcontroller/timer 50. An output pulse generator 68 provides the pacingstimulus to the patient's heart 16 through coupling capacitor 74 inresponse to a pacing trigger signal developed by digitalcontroller/timer circuit 50 each time the escape interval times out, oran externally transmitted pacing command has been received, or inresponse to other stored commands as is well known in the pacing art.

Digital controller/timer circuit 50 is coupled to an activity circuit 70for receiving, processing, and amplifying signals received from activitysensor 20. Activity circuit 70 produces an activity signal which isrepresentative of the patient's metabolic requirements. Similarly, thedigital controller/timer circuit 50 is coupled to a temperature circuit72 for receiving, amplifying and processing sensor output fromtemperature sensor 22. In the presently disclosed embodiment of theinvention, temperature circuit 72 produces an amplified, filtered analogtemperature signal which is received by digital controller/timer circuit50. The design of the temperature sensing circuitry is not critical tothe invention herein described, various suitable designs being known tothose skilled in the art. See for example, U.S. Pat. No. 4,803,987 toCalfee, et al. and incorporated herein by reference. In conjunction withADC/MUX 60, digital controller/timer circuit 50 samples and digitizesthe temperature signal from the temperature circuit 72 to obtain thedigital representation of the peak-to-peak value of the intracardiacblood temperature during each cardiac cycle. This value is provided tomicroprocessor 34, which maintains a running average over a previousnumber of cardiac cycles of the intracardiac pulse temperature.

Dynamic temperature sensor 22 is disposed in the right ventricle (RV) ofthe patient's heart to sense fluid temperature therein (RV_(temp)), andto provide a sensor output (Output_(temp)) related to changes in thefluid temperature associated with the heart's mechanical activity,contractility and blood flow. Processing by pacemaker 10 ofOutput_(temp) yields a signal which is proportional to the magnitude ofsuch RV temperature changes. Each sensed or paced RV event will yield adynamically varying signal. In the preferred embodiment, the lastsixteen valid pulse values (both paced and sensed events) are used todetermine an average temperature pulse peak value, referred to as thetemperature pulse peak average or "TEMP.AVG" Pacemaker 10 tests forvalidity of each peak temperature value on a sample-by-sample basis,based upon the requirement that a sampled temperature pulse peak valuemust be within a predetermined range defined by the TEMP.AVG value. Inthe preferred embodiment, this validity range is defined as temperaturepulse peak values between 25% to 400% of TEMP.AVG. Values outside thisvalidity range are considered artifacts and are ignored. Oncedetermined, TEMP.AVG is used to verify capture on a beat-by-beat basis.

A programmable (25% to 75%, in 12.5% steps) threshold of TEMP.AVG value(a continuous running average of 16 pulse temperature values) during awindow of time (T2) subsequent to a stimulating pulse will be used togenerate a capture detect signal in response to a successful capture dueto the stimulating pulse.

The capture detect signal is generated when the temperature circuitry 72in conjunction with the microcomputer circuit 32 generates a detectsignal during the capture detect window T2. This capture detect signalmay be used in a variety of ways, and is illustrated herein in thecontext of an auto-threshold-type pacer. In this instance, the capturedetect signal is communicated to auto-threshold logic 61. Auto thresholdlogic 61 controls the energy content of the pacing pulses delivered bythe output circuit 68 to the lead system via capacitor 74. In the eventthat a pacing pulse is delivered and no capture detect signal follows,auto threshold logic 61 will generate a control signal allowinginput/output circuit 30 to increment the amplitude of the pacing pulsesprovided by output circuit 68. Auto threshold logic 61 may alsodecrement the amplitude of the pacing pulses in response to an extendedperiod in which all pacing pulses successfully capture the heart toenable a determination of the minimum energy required to reliably pacethe heart. Auto threshold logic 61 may also respond to the failure of apacing pulse to capture the heart by quickly triggering an additionalpacing pulse at an increased amplitude.

Appropriate mechanisms for adjusting the energy content of the pacingpulses generated by output circuit 68 are disclosed in U.S. Pat. No.4,858,610 issued to Callaghan et al., U.S. Pat. No. 4,878,497 issued toCallaghan et al., and U.S. Pat. No. 4,729,376 issued to Decote, all ofwhich are incorporated herein by reference in their entireties.Alternative pacing functions which may be modified in response to thedetection or nondetection of cardiac depolarizations during the capturedetect window are described in U.S. Pat. No. 4,759,366 issued toCallaghan et al., and in the above cited U.S. Pat. No. 4,305,396 issuedto Whittkampf, both of which are incorporated herein by reference intheir entireties.

The operation of the invention is illustrated in FIG. 3. This figureshows tracings of cardiac waveforms having a unipolar pacing leadimplanted in the heart. Pacing pulses were delivered between the tipelectrode and electrode corresponding to pacemaker 10 housing (FIG. 1).

Tracing 1 was taken with the sense amplifier 64 coupled to tip and canelectrodes, and corresponds to the signals seen on the pacing lead 14.

Tracing 2 corresponds to the signal seen by the temperature circuit 72from the sensor 22.

Tracing 3 reflects the logic level output of the peak temperature detectcircuitry.

Tracing 4 corresponds to the capture detect window signal. High logiclevel signals in tracing 4 correspond to the duration of the capturedetect window T2.

Tracing 5 corresponds to the logic level output of the evoked responsedetection circuitry and indicates the occurrence of a sensed ventriculardepolarization during the T2 time window.

Tracing 6 corresponds to the output of the output circuit 68 (FIG. 2).The amplitude of the pacing pulses are reflected by the height of thepulse markers. The occurrence of pacing pulses is also reflected by theartifacts 112, 114, 116, 118 and 120 (tracing 1).

The first cardiac waveform 110 results from a normal sinusdepolarization of the heart. The sensed detect signal 122 on tracing 3reflects the normal detection of this event. In the context of the pacerof FIG. 2, this detected depolarization resets the escape interval timercontained within digital controller timer circuit 50. At the conclusionof the escape interval, timer 50 generates a V pace signal whichtriggers a ventricular pacing pulse from output circuit 68.

Artifact 112 and pacing pulse marker 142 on tracing 6 indicate thedelivery of a pacing pulse. A capture detect window is definedthereafter as indicated at 128, on tracing 4. No depolarization results,as the pacing pulse is of insufficient amplitude to capture the heart.This lack of capture is evidenced by the fact that no V sense detectsignal follows the delivery of the pacing pulse at 112. In thisinstance, the auto threshold logic 61 (FIG. 2) generates anotherventricular pacing pulse at a programmed upper rate limit interval asindicated by artifact 114. The amplitude of this pacing pulse isincreased, as indicated by pacing pulse marker 144 in tracing 6.

In this instance the second pacing pulse captures the heart as evidencedby the depolarization waveform 115 on tracing 1. This ventriculardepolarization was detected within the capture detect window 130following the delivery of pacing pulse at 114, as evidenced by V sensedetect signal 124 in tracing 3 and capture detect signal 138 in tracing5.

The tracings associated with depolarization waveform 121 illustrates asequence of three pacing pulses delivered at 116, 118 and 120. The firsttwo pacing pulses (116 and 118) fail to capture the heart, as indicatedby the absence of V sense detect signals and capture detect signalsduring capture detect window 132 and 134. Pacing pulse amplitude isincreased with each pulse, as indicated by pacing pulse markers 146, 148and 150 (all at the programmable upper rate limit interval). The thirdpulse delivered at 120 is successful in capturing the heart as indicatedby V sense detect signal 126 and capture detect signal 140 duringcapture detect window 136.

In FIG. 3, the T1 period extends from the conclusion of ventricular pacesignal depicted in the figure by pacing artifacts 112, 114, 116, 118 and120. The duration of the T1 period should be short with an expectedduration of 10-50 milliseconds. The duration of period T2 should be longenough to allow a detection of any pacemaker triggered cardiac response.The Inventor believes that 100 to 300 milliseconds is an appropriateduration for T2.

FIG. 4 shows a hardware flow diagram setting forth a state machinedescription of the detection procedure performed by the circuitry ofFIG. 2.

In state A (200) shown in the flow diagram, both the T1 and T2 timingfunctions of the capture detection timer in digital controller timercircuit 50 are disabled. This state corresponds to the pacer's operationduring sinus rhythm which inhibits the pacemaker. The state is reenteredupon the occurrence of a V sense detect signal as at 122 in tracing 3.

The occurrence of a V paced signal at decision block 202 forces a statetransition to state B (204) where the T1 timing function is enabled. Asthe period T1 times out the machine moves from state B (204) to state C(208) where the T2 window is being timed. The V sense detect signaloccurs during T2 and is taken as the indication of an evoked responseand a capture detect is declared in block 208. The expiration of the T2time period, tested at decision block 210, triggers adjustment of thepacing pulse amplitude at 212 and the return to state A (200).

What is claimed:
 1. An apparatus implanted within a patient fordetecting an evoked response of cardiac tissue evoked by a pacing pulse,comprising:pulse generator means for generating pacing pulses; means forapplying said pacing pulses to a heart; sensing means for measuringblood temperature of said patient and for generating an electricalsignal representative of said temperature; monitoring means coupled tosaid sensing means for monitoring said electrical signal provided fromsaid sensing means to detect an occurrence of a cardiac evoked response;capture detect timer means for defining a capture detect window afterthe generation of a pacing pulse by said pulse generator means; andcapture detect logic means responsive to said monitoring means and saidcapture detect timer means for detecting the occurrence of a said evokedresponse occurring within said capture detect window.
 2. The apparatusof claim 1 wherein said capture detect timer means comprises:a firsttimer means for defining a first time interval following the generationof said pacing pulse; and a second timer means for defining a capturedetect window beginning with the expiration of said first time interval.3. The apparatus of claim 2 wherein said first timer means comprisesmeans for defining a first time interval of between 10 and 50milliseconds.
 4. The apparatus of claim 2 wherein said second timermeans comprises means for defining a capture detect window of between100 and 300 milliseconds.
 5. The apparatus of claim 1 further comprisingauto threshold logic means coupled to said pulse generator means andresponsive to said capture detect logic means for altering energycontent of said pacing pulses in response to an occurrence ornon-occurrence of a detected cardiac evoked response within said capturedetect window.
 6. The apparatus of claim 5 wherein said auto thresholdlogic means comprises means for incrementing energy content of saidpacing pulses in response to the non-occurrence of a detected cardiacevoked response within said capture detect window.
 7. The apparatus ofclaim 1 or claim 2 or claim 3 or claim 4 or claim 5 or claim 6 whereinsaid sensing means comprises means for measuring blood temperaturewithin said patient's heart.
 8. A method of detecting an evoked responseof a patient's cardiac tissue evoked by a pacing pulse,comprising:generating pacing pulses; applying said pacing pulses to saidpatient's heart; measuring blood temperature of said patient andgenerating an electrical signal representative of said temperature;monitoring said electrical signal to detect the occurrence of a cardiacevoked response; defining a capture detect window after the generationof a pacing pulse; and detecting an occurrence of a said evoked responseoccurring within said capture detect window.
 9. The method of claim 8wherein said step of defining a capture detect window comprises:defininga first time interval following the generation of said pacing pulse; anddefining a capture detect window beginning with the expiration of saidfirst time interval.
 10. The method of claim 9 wherein said step ofdefining said first time interval comprises defining a first timeinterval of between 10 and 50 milliseconds.
 11. The apparatus of claim 9wherein said step of defining said capture detect window comprisesdefining a capture detect window of between 100 and 300 milliseconds.12. The method of claim 8 further comprising altering energy content ofsaid pacing pulses in response to an occurrence or non-occurrence of adetected cardiac evoked response within said capture detect window. 13.The method of claim 12 wherein said altering step comprises incrementingenergy content of said pacing pulses in response to the non-occurrenceof a detected cardiac evoked response within said capture detect window.14. The method of claim 8 or claim 9 or claim 10 or claim 11 or claim 12or claim 13 wherein said measuring step comprises measuring bloodtemperature within said patient'heart.